This invention relates generally to apparatus and methods for converting voltage magnitudes into digital counts and more particularly, but not by way of limitation, to apparatus and methods for interfacing a transducer providing a discrete analog voltage with a microcomputer or other digitally operating device.
The need for analog-to-digital conversion, whereby discrete voltage levels are converted into digital formats, is well known. For example, in digital monitoring and control systems, transducers, used to sense parameters such as temperature and pressure, provide discrete analog voltages which must be converted into digital values for use by the digital monitoring and control equipment. That is, such a transducer converts the temperature or pressure, for example, into a voltage level representative of the magnitude of the parameter. To use this voltage with a digital system which is capable of functioning only in response to two voltage levels representing either a logic zero or a logic one, the transducer signal needs to be converted into a digital format.
We are aware of two general techniques by which digital conversion can be accomplished. Both of these techniques convert the voltage from the transducer into an oscillating signal through a voltage controlled oscillator or the like. In one of these techniques, this oscillating signal gates a counter which thereupon counts another oscillating signal having a known frequency so that the count of the known frequency signal is proportional to the transducer signal. In the other technique, the oscillating signal from the transducer is counted by a counter gated by the oscillating signal of known frequency. In both of these techniques, the gating signal and the counting signal are independent; therefore, to insure accuracy, an oscillator which is stable over a long period of time needs to be used to provide the signal with the known frequency.
To obviate the necessity of using an oscillator having such long-term stability, there is the need for an improved analog-to-digital conversion technique wherein the gating and counting signals are related so that stability is required only within each counting period. It is also desirable for such a technique to provide rapid, high resolution conversion for use by high speed devices, such as microcomputers, in accurately monitoring the parameter or using the information to accurately control a system in response to the detected parameter. It is also desirable for such a technique to have a degree of programmability so that the resolution of the conversion can be controlled.